Angular rate sensors, also referred to as yaw sensors, typically include vibrating structure gyroscopes in various configurations including beams, tuning forks, cylinders, hemispherical shells and rings. The vibrating structures typically operate by taking advantage of the Coriolis effect. Yaw sensors, as well as accelerometers and other devices are increasingly being formed as micro-electro-mechanical-systems (MEMS) devices which achieve reduced size and cost.
Conventional MEMS sensors are typically made of silicon. All-silicon ring type yaw sensors have been shown to exhibit some advantages over other yaw sensor configurations. However, many such devices suffer from a fundamental problem involving how to make electrical contact with the center hub of the device, from which the resonating ring is attached and suspended.
Several solutions exist to electrically contact the center hub of the all-silicon device. One approach employs wire bonds attached directly to the center hub as disclosed in U.S. Pat. No. 6,282,958, which is hereby incorporated herein by reference. The wire bond design has several drawbacks. A misplaced wire bond may crush the springs upon which the resonating ring is suspended and render the sensor inoperable. Additionally, a wire bond made with excessive force may crush the center hub and render the sensor inoperable. Further, wire may act as an antenna and pick up undesirable stray capacitance which may be modulated by movement of the wire.
It is generally known that an all-silicon ring may be formed over a cavity that has been previously formed within a substrate. The ring and supporting springs of the MEMS structure are freed by masking the springs and then etching through the silicon device layer into the cavity, thus releasing the resonating portion of the structure. Conventional methods for forming a MEMS subsurface cavity involves bonding a wafer, that has had the cavity formed in its surface, to another wafer, and then thinning the second wafer to the desired ring structure thickness. Conventional processing of the silicon surface, beyond creating a cavity, then growing an oxide layer on that surface, generally makes the yield at the wafer bond unacceptably low. Thus, conventional yaw sensors made over a buried cavity generally do not have center hub contact runners patterned down into them, as the bond yield may be too low to produce a commercially viable sensor.
Conventional silicon ring sensors typically employ a central support hub and support beams to support the ring resonator over a cavity. The sensor also includes sense and drive electrodes which are electrically coupled to the ring resonator. A change in capacitive coupling is measured which is indicative of the sensed yaw. In manufacturing the yaw sensor, it is generally difficult to provide the proper electrical coupling to the resonator ring.
These and other drawbacks make a conventional all-silicon MEMS yaw sensor difficult to fabricate, package, electrically compensate for and pull electrical signals out of. Compensating for these and other difficulties generally make a MEMS yaw sensor more costly. It is therefore desirable to provide for a method of forming a low cost silicon MEMS device, such as a yaw sensor, that has an easy to form electrical contact. It is further desirable to provide for a MEMS device that may be formed with topside processing, in an established high volume, low cost production environment.